Recently, there have been developed optical semiconductor devices such as a semiconductor laser and an optical semiconductor amplifier equipped with an active layer having multiple quantum dots. In Patent Document 1, a quantum dot forming method is disclosed.
FIG. 1(a) is a schematic cross-sectional view of a conventional semiconductor laser, and FIG. 1(b) is a guided wave mode of light in the semiconductor laser. Referring to FIG. 1(a), an active layer 14 is provided on a lower clad layer 12 having a first conduction type. The active layer 14 is formed by stacking multiple quantum dot layers 51-55. Each of the quantum dot layers 51-55 has multiple quantum dots 41 and a base layer 40 that covers the quantum dots 41. The quantum dot densities (surface densities of quantum dots) of the quantum dot layers 51-55 are equal to each other. An upper clad layer 18 having a second conduction type opposite to the first conduction type is formed on the active layer 14.
By applying a current to flow between the lower clad layer 12 and the upper clad layer 18, emission of light takes place at the quantum dots 41 in the active layer 14, and emitted light is propagated through the active layer 14. The refractive indexes of the lower clad layer 12 and the upper clad layer 18 are smaller than the refractive index of the base layers 40 with which the active layer 14 is mainly formed. Thus, the guided wave mode of the light propagated through the active layer 14 is the strongest in the vicinity of the center of the active layer 14.